1. Field of the Invention
The present invention relates to integrated circuit packages and, more particularly, to an integrated circuit package having a heat sink integrally molded thereing for direct dissipation of heat from a semiconductor device.
2. History of the Prior Art
In the electronic industry, it is conventional to incapsulate semi-conductor devices such as integrated circuit chips in molded packages. Such packages protect the chip from the enviroment and provide means for mechanically and electrically mounting the chip for operative electrical interconnection to a circuit. Each chip comprises a number of functional components and the total number of components on a semiconductor device, known as an integrated (or "IC") circuit, has increased significantly with advances in semiconductor fabrication technology. One such advance has been the ability to reduce the physical size of the various elements of the integrated circuit chip. When this improvement is combined with the ability to increase the overall integrated circuit chip size, a number of associated problems have surfaced. For example, an increase in the number of integrated circuit elements in a chip of a given size means an increase in the electrical energy moving through the components of the chips and an increase in the heat generated by that energy. Heat dissipation from IC chips has thus become a significant consideration in electronic circuit design. As individual components, the power consumption of small electronic elements is relatively small, and the heat generated is relatively insignificant. However, as the number of integrated circuit elements located on a chip of a given size increases, the elements become more densely packed and heat generation and dissipation becomes an issue. There has also been a recent tendency toward incorporating components having higher power ratings into semiconductor devices. These factors taken collectively have resulted in heat dissipation and thermal management becoming significant concerns in semiconductor package design.
Select temperature ranges are required in the design and operation of semiconductor devices. Intense heat can cause serious degradation of portions of a semiconductor and an increase in the failure rate of such devices. For example, operational temperatures of 100.degree.-125.degree. centrigrade are generally acceptable in the semiconductor industry, and specific design features have thus been incorporated into various semiconductor devices in an effort to dissipate heat therefrom and maintain an operating temperature generally within the range.
The most conventional technique of heat dissipation in the electronic industry is the use of a heat sink. Such devices have been used for decades in basic thermal management of large electronic devices, such as dimmer switches. With the inherent complexity of small, sophisticated semiconductor devices and integrated circuit packages, there comes the need for more sophisticated heat sink designs and heat dissipation systems. One such system is set forth and shown in U.S. Pat. No. 4,888,449 to Crane et al. The Crane et al. patent teaches improvements in semiconductor packages for electrical components which includes a base member having an expanded surface area, such as grooves, to provide an additional area for heat transfer. Such designs are said to be useful in dual in-line packages (DIP) as well as plastic quad flat pack (PQFP) applications.
Another prior art approach is seen in U.S. Pat. No. 4,887,149, to Romano in which a package for semiconductor devices having a metal base plate heat sink is shown. The metal base is mounted to a suitable external heat sink designed to accommodate the neccessary heat dissipation. Such approaches are often necessary because the plastic packages which actually house the chips are generally poor heat conductors. Semiconductor packages may be produced in a number of configurations, such as plastic dual in-line packages ("DIP") and plastic quad flat packs ("PQFP"). There are also other forms of discrete semiconductor devices for which different acronyms apply. In the main, these different devices are integrated circuit chips secured within plastic packages molded from thermoset resins with a connection lead frame also molded therein.
The manufacture of a semiconductor device requires a high degree of cleanliness and sterility both in the manufacturing environment and in its ultimate environment within the plastic package. The ability of a semiconductor device to perform satisfactorily from both an electrical and a mechanical standpoint depends on the nature and quality of the materials forming the various layers of the device and their assembly within the package. For a semiconductor to operate properly, the chemical composition of these materials must be extremely pure and remain in that state. This requires that the semiconductor device, often known as a "die", remain hermetically sealed from foreign matter and, in particular, moisture. Because the die is supported within the plastic package by a metal lead frame, problems arise from the mechanical properties of the different materials. For example, the plastic to metal bond between the lead frame and the plastic package can deteriorate. When this happens, the die can be exposed to moisture and other contaminants from outside the package. In certain instances problems can even arise by diffusion of contaminants through the plastic package itself or from the gradual chemical degradation of the package. In this respect, the heat generated during operation of the semiconductor device can further exacerbate the corrosive action which may occur in the package as well as produce thermal stresses within the package and further compromise its hermetic seal. For example, the differential in the coefficient of thermal expansion between the metal lead frame and the thermoset plastic resin package can be significant. With the generation of sufficient heat by the semiconductor device to produce an operating temperature on the order of a 100.degree. centigrade, the thermal expansion incompatibility problem itself can create serious reliability issues. Although much of the heat may be transferred through the lead frame to the outside of the package, a significant portion of the heat is maintained in the package and must be dissipated directly from it. For this reason, use of heat sinks both above and below a semiconductor package has found widespread acceptance and has been the subject of considerable design attention.
U.S. Pat. No. 4,092,697 to Spaight addresses a heat transfer mechanism for an integrated circuit chip. In the Spaight patent, heat is removed from the integrated circuit chip by means of a thermal liquid material contained in a film mounted on the underside of a cover enclosing a plurality of chips. The cover is formed as a heat radiator and thus includes a cross-grooved surface forming an enlarged surface area. The thermal liquid material creates a thermal path for directly conducting heat from the die to the heat radiator as compared to conventional packages where only thermally insulative inert gas is disposed around the die. The inert gas presents a significant thermal resistance that severely limits the power dissipation of the package.
It is well known to utilize separate heat sinks that are secured directly to the semiconductor device. Various thermally conductive bonding agents may be used to attach a heat sink, and other systems may be incorporated with the heat sink to assist in heat dissipation. However, one of the primary problems in cooling an IC package is the thickness of the package itself. The molding material for plastic packages is generally a very poor thermal conductor, and while the use of a thermal film may improve performance with regard to heat transfer between the die and the package, the thickness of the package alone significantly reduces the heat transfer efficiency. In order to reduce the thickness of the overall package, it would be a distinct advantage to reduce the thickness of the portion of the plastic package material between the die and the heat sink for maximizing the heat dissipation effect thereof. The present invention addesses this problem by minimizing the distance between the die and a set of heat sink fins by integrally molding a concentrated array of heat sink fins immediately over the die in the body of the plastic package. This reduces the distance which the heat must travel to reach the fins and concentrates the heat dissipation structure within the area in which it is most critical, thereby increasing the overall reliability of the heat transfer.
It would be an advantage, therefore, to provide a semiconductor package such as a PQFP in a configuration affording improved heat dissipation to meet the ever increasing speed and power requirements for such semiconductor devices. Although plastic heat sinks have been disposed atop PQFP packages, the mismatch of thermal expansion coefficients between the interfacing materials has generated a number of structural incompatibility problems which are overcome by the present invention. In the present invention, a molded fin heat sink structure is integrally molded in the plastic package itself for direct dissipation of heat immediately above the die housed within the package.